1. Field of the Invention
The present invention relates to a battery pack which is formed by using secondary-battery cells that are repeatedly charged and discharged. Particularly, it relates to a battery pack which includes a battery-abnormality detection circuit for detecting a secondary-battery cell being abnormal.
2. Description of the Background Art
A battery pack is used as the power source of electronic equipment, such as a cellular phone and a notebook personal computer. A nickel-cadmium battery, a nickel-metal hydride battery or a lithium-ion secondary battery is used for a battery pack. In recent years, a lithium-ion secondary battery is most popular because of its light weight and large capacity.
Herein, if the voltage or the capacity of a single lithium-ion secondary battery is enough for the power required for a mobile phone or the like, a battery pack is formed by one lithium-ion secondary battery. On the other hand, a notebook personal computer or the like needs a high voltage or a great power, and thus, several secondary-battery cells are used for such equipment. They are connected in series to form a module, and then, such modules are connected in parallel. Or, in contrast, they are connected in parallel to form a module, and then, such modules are connected in series. This makes up a battery set which forms a battery pack.
In the case of such a battery set, let's assume, for example, that a minute internal short circuit is generated in one secondary-battery cell. At that time, if this secondary-battery cell subjected to the minute internal short circuit is connected in parallel with a secondary-battery cell, the latter cell's capacity is also discharged through the short circuit of the secondary-battery cell subjected to the minute internal short circuit. Thereby, the capacity of this parallel battery (i.e., the latter secondary-battery cell connected in parallel) is reduced, resulting in a drop in the battery set's capacity. If such a drop in the capacity is sharp, this parallel battery's residual capacity is zero and the other batteries (or the other secondary-battery cells) are fully charged. This disables the battery set from being charged and discharged.
In this way, in a battery pack formed by a battery set in which several secondary-battery cells are used, if a battery becomes abnormal, various methods for detecting it have conventionally been adopted.
For example, Japanese Patent Laid-Open No. 2004-31120 specification reference proposes an abnormality diagnosis apparatus. In this apparatus, a plurality of secondary-battery cells are connected in parallel. Then, several such parallel modules are connected in series to form a battery set. In this battery set, an abnormality in the cells connected in parallel is certainly diagnosed.
In addition, Japanese Patent Laid-Open No. 11-98702 specification also suggests an abnormality detection apparatus. In this apparatus, parallel modules are connected in series to form a battery set. When an abnormality is detected in charging this battery set, both an overcharge abnormality in a battery pack's parallel modules and an abnormality in a battery charger can be detected.
However, in the abnormality detection apparatus according to the prior art, an abnormality in the battery pack can be detected, but if a failure is produced in only one secondary-battery cell, the entire battery pack becomes unusable.